New Gallic union included a former emperor (8)
I believe the answer is:
caligula
'former emperor' is the definition.
'Caligula' can be an answer for 'emperor' (Roman emperor). I am not sure about the 'former' bit.
'new gallic union included a' is the wordplay.
'new' is an anagram indicator.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'included' is an insertion indicator.
'gallic' is an anagram of 'caligl'.
'caligl' enclosing 'u' is 'caligul'.
'caligul'+'a'='CALIGULA'
Can you help me to learn more?
(Other definitions for caligula that I've seen before include "Leading Roman" , "Emperor who threatened to make his horse a Consul" , "The Roman emperor said to have made his horse a consul" , "Roman emperor, notorious for his cruelty" , "Emperor after Tiberius" .)